If we were to proceed without lowercase glyphs, would it create a stumbling block for us if we decided to try to sell the fonts through a foundry or reseller?

People buy plenty of all-caps display type, and foundries sell plenty. Myfonts doesn’t seem to have any qualms.

Can anyone point us in the right direction of a standard character set for a display-only typeface

Just go for one of the >Adobe Latin character sets and drop all the unnecessary math characters and ligatures.

Can anyone point us in the right direction of a standard character set for a display-only typeface (if one even exists)?

You could do a Google search for "standard character set display typeface"... It just might turn up the character sets that different foundries use. Maybe.

If the answer is that we should definitely proceed with a lowercase, are there any examples of wide, sans-serif lowercase glyphs that could be considered elegant?

Luxury.

> We’ve also decided against small caps and other text-related character sets (tabular/lining figures, etc.) as we don’t intend this typeface to ever be used to set text.

If you want to attract a wider audience, I don't know how smart it would be to not include some of those things. Basically, if you want to sell the typeface, it's not really important what you would or wouldn't use it for, but what other people can do with it.

But it's of course your choice. Caps-only typefaces aren't that uncommon.

All Caps is fine I think. You should put duplicates of the uppercase letters into the lowercase slots of the font, not leave them blank. Leaving out letters (as opposed to mathematical symbols &c.) wouldn't be a good idea because people in non-English speaking countries may need display faces too :)

All good points, thanks for the input. We're planning on doing a ton of alternates so hopefully that will more than make up for the lack of lowercase.